1. Field of the Invention
The present invention relates generally to artificial cardiac pacemakers, and more particularly to an implantable cardiac pacemaker which generates stimulating pulses at a physiologically appropriate rate depending on the overall metabolic state of the patient.
2. Relevant Background
Since the advent of the artificial implantable cardiac pacemaker, the aims of cardiac pacing have changed from the initial goal of simply providing a lower rate limit to prevent life-threatening asystoly, to the present-day broad objective of improving the overall quality of life of the pacemaker patient. Quality of life, in this context, pertains to the performance of the heart under widely varying metabolic and hemodynamic conditions. Patients with conventional single chamber pacemakers often lack adequate heart rate and cardiac output to sustain more than slight physical exertion, and consequently suffer severe limitations on activity and fitness. For patients with complete AV block and normal sinoatrial node activity, the dual-chamber pacemaker can restore an adequate adaptation of heart rate to exercise; but that solution serves only a relatively small portion of the pacemaker patient population, and such pacemakers are susceptible to disturbances.
As a result, numerous studies have been conducted over the years seeking to uncover parameters which act internal or external to the body for possible use in controlling pacemaker stimulation rate. The goal is to control the heart rate of a pacemaker patient in a manner similar to the intrinsic heart rate of a healthy person with a normal functioning heart, under various conditions of rest and exercise; which is to say, in a physiologically appropriate manner. Parameters for controlling the pacing rate heretofore studied and proposed in the patent and scientific literature include the QT time interval, which varies with the electrical depolarization and repolarization occurring during exercise (e.g., U.S. Pat. Nos. 4,201,219 and 4,228,803); respiration rate, and thoracic impedance changes arising from increased respiration with exercise, using external adhesive electrodes and an external pacemaker (e.g., U.S. Pat. No. 3,593,718 and European patent EP-A2-0135911); the blood pH balance (e.g., U.S. Pat. No. 4,009,721 ); the central venous oxygen saturation (e.g., U.S. Pat. Nos. 4,202,339 and 4,399,820); stroke volume (e.g., U.S. Pat. No. 4,535,774); nerve activity (e.g., German patent DE 28 09 091 and U.S. Pat. No. 4,201,219); and the central venous blood temperature (e.g., German patent DE OS 26 09 365).
Applicant's German Patent No. DE 34 19 439 and related U.S. Pat. No. 4,688,573, issued Aug. 25, 1987, ("The '573 Patent") discloses techniques for rate responsive pacing which utilize both absolute temperature values and relative temperature changes of the central venous blood of the patient under various physiological conditions, and which utilize separate algorithms defining heart rate as a function of blood temperature for states of rest and exercise, respectively, together with the decision rule for selecting which of the algorithms is appropriate at any given time.
Techniques for converting mechanical forces, accelerations and pressures into electrical energy and/or signals have also been proposed in the literature for use in biomedical technology. These techniques include the generation of electrical energy to power implanted devices from piezoelectric crystals and other mechanoelectrical converters responsive to movement of the individual (e.g., U.S. Pat. Nos. 3,659,615 and 3,456,134); the use of a piezoelectric crystal embedded in silicone rubber and implanted in the pleural space between lung and ribs, to detect the respiratory rate for controlling the pacing rate (see Funke's publication in Journal Biomedizinische Technik 20, pp. 225-228 (1975)); the use of a piezoelectric sensor for measuring cardiac activity (U.S. Pat. No. 4,428,380); detecting patient activity with an implanted weighted cantilever arm piezoelectric crystal, and converting the output signal of the crystal into a drive signal for controlling the rate of a cardiac pacemaker (U.S. Pat. No. 4,140,132); and using the amplitude of a band-passed signal whose high frequency content increases with patient movement in an activity-responsive cardiac pacemaker (e.g., U.S. Pat. No. 4,428,378).
The aforementioned prior art parameters and techniques suffer various disadvantages when used in an effort to control pacemaker stimulation rate. For example, control according to the QT principle cannot distinguish emotional influences on QT interval changes from exercise-induced influences, which leads to sometimes unwanted and more pronounced emotionally-induced increases in the patient's heart rate. The change of respiratory rate with exercise varies widely between individuals, although less so with minute ventilation. Also, a person may voluntarily alter his or her respiratory rate without exercise and thereby adversely affect pacing rate. The pH level of the blood is not truly representative of patient metabolism because the significant changes toward acidity occur only at the higher levels of exercise. Similarly, changes of the central venous oxygen saturation are not a satisfactory indicator because a considerably greater decrease occurs at the beginning of exercise, even low work-load exercise, especially in those patients with limited cardiac output or tendency toward congestive heart failure, while continuing exercise may produce only slight further decreases. The stroke volume exhibits variations based on the position of the body, that is, according to whether the patient is sitting, lying or standing, which are independent of the level of exercise. The detection of patient activity by means of a neurodetector for the carotid nerve, for example, has serious limitations because of the nature of the surgery and the level of patient discomfort from this type of implant.
The detection of the activity- or motion-induced forces within or on the body by means of a piezoelectric crystal, a microphone or other mechanoelectrical transducer exhibits the desirable characteristics of a fast response to the onset of exercise, but has certain serious disadvantages including the deleterious effect of noise disturbances external to the body, such as from nearby operating machinery, or emanating within the body, such as from coughing, sneezing, laughing, or the like. Accordingly, disturbances unrelated to exercise can affect the heart rate, when accelerometer-type detection is utilized for control of the pacemaker stimulation rate.
It has been assumed in the prior art that the maximum acceleration values detected by an activity-controlled cardiac pacemaker in a patient undergoing exercise occur in the range of the resonant frequency of the major body compartments such as the thorax and the abdomen, i.e. approximately 10 Hz (e.g., see Proceedings of the European Symposium on Cardiac Pacing, editorial Grouz, pp. 786 to 790, Madrid, 1985). Thus, the prior art teaches that the maximum sensitivity should be in the range above 10 Hz (e.g., see also, Biomedizinische Technik, 4, pp. 79 to 84, 1986, and the aforementioned U.S. Pat. No. 4,428,378).